Generally, when making an injection molded product using a synthetic resin as virgin material, if the desired injection molded product is a hollow molded product with a complicated structure, the molded product can be made by the following processing steps. Firstly, a water soluble resin is injection-molded into a core with a predetermined shape and the molded core is then positioned on a die for producing the desired product. Consequently, water insoluble resin is overmolded on the molded core that is positioned on the die. After the water insoluble resin solidifies, the water soluble resin core is sprayed with water to dissolve it. As a result, the desired hollow product made of a water insoluble synthetic resin is produced.
The above process is generally known as the "lost core technology." Typical water soluble resin used for making the core includes GBC 1330 TE (trade mark) which is available from Belland AG (Switzerland). GBC 1330 TE is a resin which is produced by mixing PAEA (polyacrylic acid-ethyl acrylate) and copolymers of acrylic acid and ethyl acrylate at a predetermined ratio and with a predetermined amount of mineral filler and, which is referred to as virgin resin. The copolymer has a hydrophilic group and, hence, the polymer chain is dissolved by water but not decomposed in water. Therefore, a polymer suspension is produced and it can be substantially recycled for reuse.
With the lost core technology, however, using the water soluble resin to injection-mold the core results in residual products, such as the polymer suspension produced during the dissolution of the core, that are discharged through the sewage system. In this case, even if the polymer suspension used is not noxious, it leads to environment contamination, particularly water pollution.
To solve the water pollution problem, a method for recycling residual resin from the polymer suspension in addition to the product molding process of the lost core technology has been proposed by Belland AG. More specifically, the proposed resin recycling method comprises either of two core dissolving steps.
Firstly, water is pumped passed and through the core that is positioned in a closed circuit. In this step, the closed circuit system is used to recirculate water having a temperature of 100.degree. C. or more to promote the dissolution of the core.
Secondly, an opened circuit system is used to spray several part-branched high temperature water streams from an inner part thereof and to drain the water to a sump in order to recycle the residual resin. Here, the time for dissolving the core depends on the thickness of the core, the shape of system parts and the temperature of water but is preferably set to fifteen minutes, and the specific dispersion time is set to 2-5 mm/min at a water temperature of 95.degree. C.
When the polymer suspension produced in the core dissolving process is subject to thin film evaporation, water is evaporated and the remaining resin is vacuum dried to obtain a pellet-shaped product which can be recycled by mixing with the core virgin material at an appropriate ratio.
However, with the first recycling method of Belland AG wherein water is pumped around the core in the closed circuit, although the melting temperature of resin is between 180.degree. C. and 210.degree. C., because the temperature of water to be used is more than 100.degree. C., the resin can only be dissolved slightly in the water, and moreover, since the boiling point of water is 100.degree. C., the water used to dissolve the core as well as the polymer are boiled at a temperature above 100.degree. C. Therefore by boiling the water or polymer suspension, the composition of the polymer suspension is changed, or bubbles and foam are produced within the tank containing the polymer suspension. The result is that the pumping efficiency is lower, the time for dissolving the core is longer in accordance with the compositional change of the polymer suspension or the lowering of pumping effect, and the specific dispersion time is also longer.
On the other hand, with the second recycling method, a higher pressure of 10-15 kg/cm.sup.2 is needed to spray the polymer suspension to dissolve the core. As a result, energy consumption is considerably increased and the cost of manufacturing the product is also increased.
Further, the separation of water from the polymer suspension is accomplished by a heating process wherein the polymer suspension is heated to evaporate the water component. Accordingly, the physical property of the resin is probably changed by the heat produced in the heating process, so that the physical property of the virgin resin is also changed.